Corresponding arrangements are known by which each supporting pin is provided inside said control chamber with a pinion engaging a gear rim of a control wheel rotatably mounted within said control chamber and rotatable by adjustment means outside said chamber. Such a device demands a very exact montage of each part involved since it is of uttermost importance that after each adjustment all the blades take up the same relative angular position. Further the outer means for the adjustment have to be arranged outside the backwall. Consequently, such an arrangement demands quite a lot of space.
Therefore, more commonly arrangements as the one first stated above are used. By the known arrangements of this type the arms on the supporting pins are nearly radial and their studs engage slots in an annular disc rotatably mounted within the control chamber and connected to adjustment means outside the chamber by which it can be given a limited rotation resulting in a uniform pivoting of all arms and thereby all guide blades. In such an arrangement an exact uniform adjustment of the angular position of each guide blade mainly depends on the precision at the manufacture and is easier to ensure than by use of gears. Also such an arrangement demands, however, that the outer adjustment means are mounted at the outside of the backwall as well as special supporting means for the control member and, therefore, also such an arrangement demands relatively much space making it difficult to obtain space enough when the arrangement is to be used in more compact machinery such as multistage compressors and compressors combined with a gear device, where the impeller is arranged overhanging its driving shaft.